Due to rapidly increasing traffic in optical communications in recent years, the transmission capacity of presently used single-core optical fibers is approaching its limit. So, as a means to further expand the communication capacity, multicore fibers in which a plurality of cores are formed in one fiber have been proposed. Use of multi-core fibers can suppress the laying cost of optical fibers and expand the transmission capacity.
When a multi-core fiber is used as a transmission path, each of the core parts of the multicore fiber needs to be connected to another multi-core fiber, an optical element, or the like respectively to send and/or receive transmission signals.
On the other hand, a multi-core fiber has cores arranged at positions other than the center of the cross section, and this leads to a problem that the connection thereof is more difficult compared to connection of single-core fibers.
When connecting such multi-core fibers, it is preferable that the arrangements of the cores of the multi-core fibers can be easily identified.
As such a multi-core fiber, the prior art (e.g., JP A 2011-170099) discloses a multi-core fiber provided with a marker on a cladding thereof for identifying core arrangements.
Moreover, besides multi-core fibers, when compared with connections of commonly used single-core optical fibers, it is still difficult as in the case of multi-core fibers to connect optical fibers in which a form of a cross section of the optical fiber taken in a perpendicular direction to a longitudinal direction thereof has an orientation in a rotational direction with the longitudinal direction of the optical fiber as an axis (such as eccentric core fibers or polarization-maintaining fibers).
However, with the method disclosed in JP A 2011-170099 in which a marker with different refractive index is provided inside the cladding cross section, it is impossible to identify the core arrangement unless the cross section is cut or a resin coating of side surface of the cladding is removed and the position of the marker is observed under a microscope. Thus, it is difficult to easily identify the core arrangement from appearance.